Insulated alkali metal conductor



6, 1969 A. GUTTAG INSULATED ALKALI METAL CONDUCTOR 2 Sbeets-Sheet 1 Filed Sept. 25, 1967 1/ III/ [1/1111 YIIIIII/l/ [/1 I R AW m? Mm E 0 W U f v/ W a BY zmw z/ Aug. 26, 1969 A. GUTTAG INSULATED ALKALI METAL CONDUCTOR N 2 Sheets-Sheet 2 Filed Sept. 25, 1967 ATTORNEY WM W@ United States Patent Jersey Filed Sept. 25, 1967, Ser. No. 670,166 Int. Cl. H01]: 7/28 U.S. Cl. 174-120 15 Claims ABSTRACT OF THE DISCLOSURE Electrical conductors comprising an alkali metal conducting core are coated with a preformed heat shrink able inert olefin hydrocarbon polymer film or sheet and the hydrocarbon shrunk into tight engagement with the alkali metal. The heat of the molten metal or of the metal while it is cooling can be used to shrink the polymer.

An alkali metal conductor having a coating of an inert olefin hydrocarbon polymer has the surface of the coating modified to render it receptive to a vinylidene chloride polymer and then a vinylidene chloride polymer is directly integrated into said modified surface to render the olefin hydrocarbon polymer substantially impervious to oxygen and carbon dioxide.

The present invention relates to electrical conductors comprising a conducting element of an alkali metal continuously surrounded by a layer of a normally solid hydrocarbon polymer.

Recently it has been proposed in Humphrey Patents 3,333,037 and 3,333,049 to prepare alkali metal conductors having a covering of an olefin polymer. The polymer can be preformed in tubular form and the molten sodium carefully fed into the tube. More preferably the molten hydrocarbon polymer is extruded around the alkali metal, preferably sodium, while the molten metal is also being extruded. This procedure, however, presents several problems. In the first place the molten polymer as it is extruded is very weak and breaks can occur. Secondly, the molten polymer has a softening point above 100 C., generally at least 105 C. and often higher, whereas sodium has a solidification point of 97.8 C. As is well known sodium contract-s considerably as it is cooled. Thus the density of sodium increases as it goes from a liquid at about 97.8 C. with a density of 0.929 to a solid at 97.8 C. with a density of 0.952 and to a solid at 20 C. with a density of 0.971. As a result there is a tendency for a space to form between the solidified sodium and the surrounding polyethylene or the like coating, e.g. in tubular form. Furthermore regular polyethylene has a tendency to tear easily and has a relatively low tensile strength, e.g. around 2000 to 2600 p.s.i. at room temperature while the tensile strength is reduced sharply at elevated temperatures. Humphrey Patent 3,333,049 discloses that after the polyethylene tube is filled with sodium the product can be passed through a series of dies on a conventional wire drawing machine to increase the ultimate tensile strength of the conductor. This process of course has the danger that the polyethylene film might break during the drawing operation particularly since the thickness of the polyethylene covering is reduced.

It has also been proposed in Humphrey Patent 3,333,- 050 to extrude a monoolefin polymer, e.g. polyethylene, around sodium to form a coating or liner around the sodium and to extrude a polymer reactive with sodium around the polyethylene. The reactive polymer can be a material such as polyvinyl chloride or can be an olefin polymer, e.g polyethylene containing a crosslinking agent 3,463,872 Patented Aug. 26, 1969 such as a peroxide which is reactive with sodium. The Humphrey patent shows that to cure the peroxide treated polyethylene it is necessary to employ an oven at a temperature of 190 C. for at least 10 minutes. This of course is well above the softening point of polyethylene and consequently problems are encountered in retaining the initial liner uniformly .around the sodium. The problem is increased by the fact that the temperature of curing is'also well above the melting point of the sodium and not only will the sodium expand about 50% but form unstable sodium will be within form unstable polyethylene. Furthermore .it is necessary by such .a procedure to form a laminate with the attendant dangers of delamination.

When using an all hydrocarbon insulation there is the further problem that it is relatively porous to oxygen, av gas known to be reactive with sodium and other alkali metals. Thus low density polyethylene (0914-092) has amoxygen permeability of 500 expressed as cc./ sq. in./rnil/24 hours at 25 0, medium density polyethylene (0.93-0.94) has a permeability of 535, high density polyethylene (0.95-0.96) has a permeability of 185 while polypropylene (unoriented) has a permeability of 70 and 370 oriented. On the same scale the carbon dioxide permeability for the low density polyethylene is 2700, medium density polyethylene 2500, high density polyethylene 580, polypropylene (unoriented) and oriented polypropylene 180.

Accordingly it is an object of the present invention to insure that a monoolefin polymer coating for an alkali metal adheres to the metal without any intervening air space.

A further object is to reduce the risk of breakage of an insulating hydrocarbon covering for an alkali metal electrical conductor.

An additional object is to prepare an alkali metal electrical conductor with an olefin polymer covering having reduced permeability to oxygen and carbon dioxide.

Yet another object is to provide an alkali metal electrical conductor with an olefin polymer covering as a protective layer for an insulating layer reactive with the alkali metal while at the same time avoiding any possibility of delamination between the olefin polymer and the reactive layer.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

It has now been found that these objects can be attained in several ways as more fully set forth below.

The entire disclosure of the Humphrey Patents 3,333,- 037, 3,333,049 and 3,333,050 is hereby incorporated by reference.

While the following description will make reference to sodium as the alkali metal, and this is the preferred form of the invention, the invention is also applicable to the other alkali metals having an atomic weight of less than 40, i.g. lithium and potassium. metals having an atomic weight of less than 40, i.e.

According to one aspect of the invention the alkali metal electrical conductor, e.g. sodium, is provided with an insulating layer of a preformed heat shrinkable oriented monoolefin polymer. The orientation can be uniaxial or biaxial, most preferably biaxial, As the monoolefin polymer there can be used polyethylene, polypropylene, ethylene-propylene copolymer (e.g. 50:50 by weight), ethylene-neohexane copolymer (e.g. 90:10 by weight), ethylene-decene copolymer (95:5 by weight), ethylene-amylene copolymer, ethylene-butylene copolymer polyamylene, poly Z-methylamylene, propyleneamylene copolymer, etc.

The preferred polymers are polyethylene (density 0.91-0.96) and polypropylene (density 0.90-0.91). The monoolefin polymer most preferably is crosslinked prior to orientation. Thus crosslinking of polyethylene, polypropylene and the other polymers can be accomplished by using inorganic or organic peroxides such as hydrogen peroxide, dicumyl peroxide, dibenzoyl peroxide, di-t-butyl peroxide, and other conventional peroxide curing agents. More preferably, however, the crosslinking of polyethylene or polypropylene is accomplished by irradiation to a dosage of 2 to 50 megarad, more preferably 6 to 20 megarad. The crosslinked polyethylene, for example, is

4 radiated and racked polyethylene over the extruded polyethylene per se are self-evident. In addition the irradiated polyethylene while being heat scalable in the range of 150315 C. did not soften or melt at such temperatures.

For best results the oriented polyethylene, polypropylene or other monoolefin polymer should have a shrink energy at 96 C. of at least 50 and can be higher, e.g. up to 500 p.s.i.

While irradiation is essential for maximum development of properties with polyethylene, such is not the case with polypropylene.

While the stretching can be uniform in the longitudinal and lateral direction this is not essential. Typical examples of suitable oriented monoolefins for use in the invention are given in Table 2. Where both irradiation and racking were employed the irradiation preceding the racking.

TABLE 2 Irradiation Racking Transverse Longitudinal No. Polymer Denslty megarad tcmp., 0. Stretch ratio stretch ratio 1 Polyethylene 0. 916 12 96 5 3 2 d 0. 916 12 125 4 3 0. 960 10 175 4 3 0. 916 50 90 3 2 0.910 8 00 5 4 0.916 B8 4 3 7.. 0. 910 12 96 (i 3 8 d0 0. 925 12 110 5 3 9 Polypropylene... 0.90 2 140 0 3 10. Polyethylene 0. 916 10 93 3. 5 3. 5 11, do 0. 916 12 90 3 3 12. 0. 016 12 96 4 5 13. 0. 916 15 98 5 3 14..-. ...(lo 0.916 12 06 3. 5 3.5 15 ..do 0.916 None 88 2 2 16 Polypropylene 0.90 None 140 8 3 17 do 0. 90 None 150 7 3 more flexible than conventional polyethylene. Additionally it does not melt or soften at temperatures of 170 C. and higher and there is no danger of it softening when in contact with the molten sodium which comes out of the extruder.

The olefin polymer, whether crosslinked or not can be stretched in conventional fashion, e.g. by blowing a bubble. The stretching can be to an extent of 20%, 200%, 300%, 600% or 700% or higher in either one direction (uniaxial orientation) or two directions (biaxial orientation). Preferably the stretching is 100 to 600% and is biaxial. The shrinkage capability of the oriented polymer can be from 3 to 75% at 97.8 C. and is preferably 10 to 55% at that temperature. As is known in the art the amount of shrinkage in a heat shrinkable polymer increases as the polymer is heated toward the temperautre at which it was initially stretched. Thus a sample of polyethylene (0.9l4 density) was irradiated with 20 megarad, heated to C., stretched from a thickness of 35 mils to 14 mils and cooled under tension. The product when reheated to 79 C. showed no shrinkage, when reheated to 93 C. showed 5.9% shrinkage and when heated to 107 C. showed 44.1% shrinkage. (All samples were reheated for the same time, namely 15 minutes.)

The effect of a second stretching by blowing a bubble (racking) and irradiation (12 megarad dosage) on extruded and hot blown conventional polyethylene is shown in Table 1 using the same polyethylene of 0 .96 density.

The shrink energy is the force of contraction at a given temperature when the material is restrained. The improved high temperature properties of the hot blown, ir-

The shrink properties of some of the polymers set forth in Table 2 are given in Table 3.

Polymer No. 16 (polypropylene) had a shrink of only about 34% in either direction at 96 C. but had a shrink between 40 and 60% in either direction at C.

The heat shrinkable monoolefin polymers can be employed as insulation in a thickness of 2 mils or more, e.g. up to 200 mils or more. The crosslinked polymers, e.g. irradiated polyethylene, because of their increased tensile strength and tear resistance and non-melt properties are especially valuable because these properties allow the insulation to be thinner than when conventional polyethylene (even though oriented) is employed. Thus good insulation can be obtained at 3 mil thickness of the crosslinked heat shrinkable polyethylene though higher thicknesses such as 5 mils, 10 mils, 15 mils or even higher thicknesses can be employed with this and the other heat shrinkable monoolefin polymers employed in the present invention.

The alkali metal conductor generally can have a diameter of 10 to 500 mils or more, e.g. as a cylindrical rod.

The heat shrinkable electrical insulator can be preformed in tubular form and the molten alkali metal, e.g. sodium, flowed into the tube, e.g. at a temperature of 98 to 200 C. The heat of the alkali metal will cause the heat shrinkable polymer to shrink in tight physical engagement with the metal and there will be no chance for vacant space to occur between the insulator and the conductor. Since the polymer is a poorer conductor of heat than the metal the polymer will cool more slowly than the metal. This aids in ensuring that the polymer remains in tight engagement with the metal while the latter cools and contracts. If crosslinked heat shrinkable polymer is used there is no danger of melting the polymer. When thermoplastic polymers are employed, e.g. irradiated polyethylene or polypropylene, the shrinking of the polymer should be carried out below the melting or softening point of the polymer. Thus molten sodium at 100 C. can be poured into irradiated biaxially oriented polypropylene or unirradiated uniaxially oriented high density (0.96) polyethylene tubing and the tubing will heat shrink about the metal into tight continuous physical engagement therewith.

The sodium in another example was cooled to room temperature and the solid sodium then introduced into a tube of irradiated biaxially oriented polyethylene (stretched at 90 C. 3:1 longitudinally and 3:1 laterally after irradiation to a megarad dosage). The tube had a diameter 10% greater than that of the sodium rod. After introducing the sodium the tube was heated to 90 C. with a blast of hot nitrogen from an air gun to shrink the tube in tight continuous physical engagement with the sodium.

One of the advantages of the present invention is that the heat shrinkable jacket if applied as a preformed tube can be larger in diameter than the sodium rod which is being extruded and there is no need to maintain a narrow pressure range on the alkali metal in order to maintain the shape of the tubing.

For safety purposes wherever the sodium or other alkali metal is exposed to the surrounding atmosphere the atmosphere is inert, e.g. an inert gas such as nitrogen, helium, argon or neon. When a coolant is employed it also should be inert to the alkali metal and the insulation. Thus it can be an inert gas as specified above or a hydrocarbon oil, e.g. an aromatic or aliphatic hydrocarbon or cooling can be accomplished by passing a heat exchange liquid countercurrent through a tube adjacent to the plastic tube containing the sodium or other alkali metal.

To reduce the oxygen and carbon dioxide permeability of the hydrocarbon polymers, and to a lesser extent to reduce the water vapor permeability, the hydrocarbon insulation can have an integral continuous coating of saran on the outside surface (i.e. the surface not in contact with the alkali metal conductor). As the insulation which is integral with the saran there can be employed any of the heat shrinkable monoolefin polymers set forth above or there can be used a non-heat shrinkable monoolefin polymer such as the polyethylene, polypropylene or the like obtainedby extruding the monoolefin polymer around the alkali metal conductor as shown in Humphrey Patents 3,333,050; 3,333,049 and 3,333,037.

Unless otherwise indicated all parts and percentages are by weight.

By the term saran there is intended as is well known in the art vinylidene chloride polymers containing at least of vinylidene chloride and preferably at least but not over of vinylidene chloride. There can be used copolymers, terpolymers, tetrapolymers and the like. Typical examples of copolymers are vinylidene chloride-acrylonitrile (:25, :20, :15), vinylidene chloride-ethyl acrylate (80:20), vinylidene chloride-vinyl chloride (80:20), vinylidene chloride-vinyl chloride-dimethyl maleate (75 :2025), vinylidene chloride-acrylonitrile-isobutylene (70:25z5), vinylidene chloride butyl methacrylate (:10), etc.

The saran coating cannot be applied in the manner set forth in Humphrey Patent 3,333,050 for other coatings since the thus extruded saran will not be integral with the polyethylene and there will be the problem of delamination pointed out previously.

Instead the saran can be applied as a solution in a solvent such as acetone, methyl ethyl ketone, tetrahydrof-uran, methyl isobutyl ketone, ethyl acetate, butyl acetate, amyl acetate, nitromethane, nitroethane, 2-nitropropane, etc. The saran is normally present as a 10-20% solution but can be present in lesser amount or in some instances in greater amount.

The saran is preferably employed as Saran F-120 (a vinylidene chloride-acrylonitrile copolymer 80:20 having a viscosity of 200- cps. as a 20% acetone solution) as a 15% solution in a mixture of acetone and methyl ethyl ketone (3:1).

The monoolefin polymers, e.g. polyethylene and polypropylene (whether irradiated or not and whether heat shrinkable or not) are normally not receptive to receive the saran. Accordingly the surface of the monoolefin polymer must be modified as is known in the art to render the surface receptive. Such modification can be done by treating the surface of the polyethylene or polypropylene (whether irradiated or not and whether heat shrinkable or not) with an oxidizing means, e.g. of the oxygen type such as chromic acid, aqua regia, sodium dichromate, potassium permanganate, sodium permanganate or an oxidizing gas flame or by treatment of the surface with corona discharge. Of course if heat is employed in the treatment of heat shrinkable polymer, e.g. an oxidizing gas flame is employed, the polymer should be maintained under tension until it has cooled below the shrink temperature. The time for this treatment is normally very brief since film travelling at a rate of to 600 ft./min. can pass under an oxidizing gas flame or under corona discharge for only a fraction of a second and have an adequate treatment.

After the surface of the monoolefin polymer has been modified in the manner indicated above (the preferred modification being with corona discharge) the surface is coated with the saran solution in a volatile solvent in order to provide the monoolefin polymer with a vinylidene chloride polymer (saran) directly integrated with the modified surface of the monoolefin polymer. The solvent is then removed, e.g. in an oven. The saran can be applied to the monoolefin polymer (e.g. polyethylene either irradiated or unirradiated) prior to application of the polymer as insulation to the alkali metal. Alternatively the saran can be applied to the monoolefin polymer after the polymer has been applied as insulation to the alkali metal. In the latter case the solvent employed preferably should be removable at a temperature below the melting point of the metal, e.g. sodium. Thus solvents such as acetone, methyl ethyl ketone and tetrahydrofuran are quite suitable for such purpose since they all boil below 95 C. The solvent appears to help the saran penetrate slightly into the monoolefin polymer surface and thus obtain the directly integreated or merged relationship in contrast to a simple lamination.

The saran coating desirably can be from 0.075 mil to 1.5 mil but in no case should be more than 10% of the thickness of the monoolefin polymer. This is important for several reasons. Thus there is no danger of the saran penetrating to the extent that it will contact the sodium with which it is reactive. Furthermore, if the saran coating is too thick the low temperature flexibility of the insulation is greatly reduced and in addition the insulation tends to be too brittle.

Preferably the saran coating is about 0.1 to 0.3 mil thick. Despite the extreme thinness of the saran coating it has :a great effect on the oxygen and carbon dioxide transmission of the coating. Thus a 0.2 mil integral Saran F- coating on a 5 mil film of polyethylene (10.914 density) insulation coating on sodium reduced the oxygen permeability of the polyethylene to 20% of its former value, and reduced the carbon dioxide permeability to 10% of its former value. A 1.5 mil integral coating of Saran F-120 (stretched 300% in each direction) heat shrunk on sodium on a 15 mil sheet of biaxially oriented polypropylene reduced the oxygen permeability of the polypropylene to less than 10% of its former value.

A 0.2 mil integral Saran F-120 coating on a 3 mil film of biaxially oriented irradiated polyethylene (0.914 density, irradiated to an extent of 8 megarad having a shrink energy at 96 C. of 300 p.s.i. and having a 45% transverse shrink at 96 C. and a 35% longitudinal shrink at 96 C.) insulation around a sodium rod reduced the oxygen permeability of the irradiated polyethylene to 15% of its former value and reduced the carbon dioxide permeability to about 6% of its former value. In every instance there was also a noticeable reduction in the water vapor permeability although this change was minor compared to the reduction in the tendency of the monoolefin polymer to be permeated by oxygen and carbon dioxide. Even when the integral saran coating is applied to a heat shrinkable polymer the saran is not removed in the subsequent heat shrinking. This is apparently due at least in part to the thinness of the saran layer.

The invention will be understood best in connection with the drawings wherein:

FIGURE 1 is a partial schematic diagram of applying a heat shrinkable film to an alkali metal conductor according to the invention;

FIGURE 2 is a sectional view along the line 22 of FIGURE 1;

FIGURE 3 is a view partially in section of an alternative method of forming heat shrinkable tubing around an :alkali metal conductor according to the invention;

FIGURE 4 is a sectional view of an alkali metal conductor having a dual coating according to the invention;

FIGURE 5 illustrtaes the heat shrinking of a tube of monoolefin polymer about a rod of alkali metal;

FIGURE 6 illustrates one method of forming an integral saran coating on a monoolefin polymer insulation for an alkali metal; and

FIGURE 7 is a sectional view along the line 7-7 of FIGURE 6.

Referring more specifically to FIGURE 1 of the drawings irradiated (8 megarad) polyethylene (0.916 density) tubing 2 is passed into chamber 4. The lower portion of the chamber contains an inert liquid specifically mineral oil 6 heated to 99 C. and the upper portion is filled with nitrogen gas. Through the center of the tubing is a pipe 8 filled with molten sodium. The irradiated polyethylene tubing was fed from feed rolls 10 and 12 to a pair of upper rolls 14 and 16. Nitrogen gas was introduced in known fashion into the tubing and used to maintain the bubble between the feed and upper rolls. The irradiated polyethylene film was cooled as it passed into the nitrogen atmosphere from the surface of the hot bath to the deflated rolls. The molten sodium was under sufficient pressure to fill the expanded diameter of the pipe 8 as illustrated at 18. Feed rolls 10 and 12 were rotated at a surface speed of 18 ft./min. and upper rolls 14 and 16 at a surface speed of 63 ft./min. The transverse stretch was 3.5 to 1 and the longitudinal stretch was also 3.5 to 1. The finished film had a thickness of 5 mils, a tensile strength at 93 C. of 2000 p.s.i., a shrink at 96 C. of

40% in the transverse direction and of 40% in the longitudinal direction and a shrink energy at 96 C. of 300 p.s.i. The molten sodium in the pipe 8 had cooled sufficiently before it emerged from the pipe that it was a solid with a temperature of about 93 C. as it emerged to form rod 20. While the heat shrinkable polymer produced by such stretching was restrained from shrinking while it was under tension between the feed rolls and the upper rolls it was no longer under tension after it left the upper rolls. The residual heat in the solid sodium was suflicient that the heat shrinkable irradiated polyethylene shrunk into tight, continuous engagement with the sodium at approximately the point 22.

As shown in FIGURE 2 the solid sodium metal 20 of a thickness of 65 mils had a continuous partially heat shrunk irradiated polyethylene insulation 24 having a wall thickness slightly over 5 mils in tight engagement therewith. The polyethylene was only partially heat shrunk since the heat stretched biaxially oriented polyethylene tube diameter as formed was only slightly larger than the diameter of the sodium rod. Consequently when the polyethylene was heat shrunk there was considerable residual shrink left therein. In those cases where the sodium in its further cooling to room temperature contracts away from the polymer the heat shrinkable polymer can be briefly heated with a blast from a hot air gun. Such heating is so short that while the polymer will shrink the sodium will not be heated.

When employing polypropylene which is normally on. ented either uniaxially or biaxially by heat stretching at -150 C., e.g. C. the amount of shrinkage below the melting point of sodium is usually so small that the heat shrinkable polypropylene tubing as it emerges past the upper rolls 14 and 16 (in a device as shown in FIG- URE 1) is heated briefly with an air gun, e.g. at point 22, to insure shrinkage of the polypropylene into tight engagement with the sodium metal.

As shown in FIGURE 3 molten sodium from source 26 is passed via pipe 28 into tube 30 of irradiated biaxially stretched polyethylene (8 megarads irradiation, stretched 300% longitudinally and 250% laterally). Tube 30 is formed from 5 mil film 32 in conventional fashion by heat sealing the overlapping ends of the film as at the line 34. The pipe 28 terminates in the box 36 which is filled with nitrogen gas and the molten sodium flowing from the end of the pipe is sufficiently hot to cause the tube 30 to shrink into tight, continuous engagement therewith although the tube still has much residual shrink left therein. (If desired in this form of the invention the sodium can be converted to solid form in the pipe 28 prior to emerging into the formed tube 30.)

The tube 30 and the sodium rod are cooled to room temperature with the aid of coolant, e.g. mineral oil, in coolant bath 38. To insure tight engagement of the tube to the sodium after cooling the heat shrinkable tube is heated very briefly with a blast of hot air from a ring jet 40. The heating period is so brief that there is no appreciable warming of the sodium. The polyethylene film has a thickness of 10 mils and the sodium rod formed a diameter of 167 mils.

In place of the biaxially oriented irradiated polyethylene there can also be employed for example uniaxially oriented polypropylene (stretched laterally) in this form of the invention.

FIGURE 4 shows in section a product formed by the procedure described in connection with FIGURE 3 but employing as the starting film rather than irradiated polyethylene a 5 mil film of biaxially oriented irradiated polyethylene having an integral 0.2 mil coating of saran. As shown in FIGURE 4 the outer 0.2 mil film 42 of saran (vinylidene chloride-acrylonitrile 80:20) is integral with and merged into the irradiated, biaxially oriented polyethylene (6 megarads irradiation and stretched 300% in both directions) which is in partial heat shrunk tight engagement with 63 mil diameter sodium rod 46.

As shown in FIGURE 5 there is provided preformed heat shrinkable irradiated biaxially oriented polyethylene film 48 of 10 mils thickness and a diameter of mils and a solid rod 50 of sodium of 152 mils. The film is heat shrunk by passing through ring heater 52 having a temperature of 92 C. so that the film partially shrinks into tight engagement with the sodium. The entire apparatus is preferably maintained in an atmosphere of nitrogen. The insulation can extend beyond the sodium and the ends of the insulation also heat shrink to completely seal the ends of the sodium.

As shown in FIGURE 6 irradiate (8 megarad) biaxially oriented polyethylene tubing 54 which has been heat shrunk into tight engagement with solid metallic sodium rod 56 is passed continuously through corona discharge apparatus 58 to render the external surface of the tubing receptive to saran. Then the tubing is passed through bath 60 filled with a 15% solution of Saran F-120 in a mixture of acetone and methyl ethyl ketone (3:1) at room temperature. The tubing with a coating of saran is then passed through oven 62 at 80 C. to remove the solvent. The saran formed a continuous coating which merged into the irradiated polyethylene. The saran coating had a thickness of 0.2 mil, the irradiated biaxially oriented polyethylene a thickness of 6 mils and the sodium metal a diameter of 112 mils.

Greater thicknesses of the saran layer can be built up by repeated passages through the saran bath.

In the form of the invention illustrated in FIGURE 6 instead of employing heat shrinkable monoolefin polymer there can be used non-heat shrinkable monoolefin polymer such as the regular 0.93 density polyethylene extruded as shown in FIGURE of Humphrey Patent 3,333,049. However, as previously. indicated, preferably there is used the heat shrinkable -monoolefin polymers. In the examples illustrated by the drawings the irradiated polyethylene had a density of 0.916 and the polypropylene had a density of 0.90. However, as previously pointed out other heat shrinkable polyethylenes and polypropylenes can be used.

The metallic sodium conductor as indicated can be circular, hexagonal, rectangular or of other cross-section.

As used in the present claims hot metal is intended to include metal having a temperature of 50 C. or above, e.g. 90 0., 100 0., 200 C. or even higher.

What is claimed is:

1. An electrical conductor having as essential components thereof an electrically conductive member comprising a solid alkali metal core and a flexible heat shrinkable hydrocarbon polymer layer of a monoolefin having from 2 to 6 carbon atoms, electrically insulating and surrounding said conductive member, said polymer layer beingoriented in at least the lateral direction and in partial heat shrunk condition and in tight, continuous physical non-adhesive engagement with said 'core.

2. An electrical conductor according to claim 1 wherein the metal core is sodium.

3. An electrical conductor according to claim 2 wherein the polymer layer is a crosslinked polymer of a monoolefin having 2 to 3 carbon atoms.

4. An electrical conductor according to claim 3 wherein the crosslinked polymer is polyethylene irradiated to an extent of 2 to 50 megarad.

5. An electrical conductor according to claim 4 wherein the irradiated polyethylene is biaxially oriented.

6. An electrical conductor according to claim 2 wherein the polymer layer is uncrosslinked polypropylene.

7. An electrical conductor according to claim 1 wherein the polymer layer is integrally coated with saran and the said saran coating has a thickness not over 10% of the thickness of the said polymer layer.

8. An electrical conductor according to claim 7 wherein the metal is sodium.

9. An electrical conductor according to claim 7 wherein the polymer layer is a crosslinked polymer of a monoolefin having 2 to 3 carbon atoms.

10. An electrical conductor according to claim 9 wherein the monoolefin polymer is polyethylene irradiate to an extent of 2 to megarad.

11. An electrical conductor according to claim 10 wherein the irradiated polyethylene is biaxially oriented.

12. An electrical conductor according to claim 7 wherein the polymer layer is uncrosslinked polypropylene.

13. An electrical conductor according to claim 9 wherein the polymer is polyethylene.

14. An electrical conductor according to claim 9 wherein the polymer is polypropylene.

15. An electrical conductor according to claim 9 wherein the saran coating has a thickness of 0.1 to 1.5 mils.

References Cited UNITED STATES PATENTS 2,286,759 6/ 1942 Patnode.

2,929,744 3/ 1960 Mathes.

3,093,448 6/1963 Kirkpatrick.

3,269,862 8/1966 Lanza l172l8 3,333,050 7/1967 Humphrey.

OTHER REFERENCES Alphlex Shrinkable Tubing, ad. brochure, August 1961.

Birks, F. B.: Modern Dielectric Materials, London, Heywood & 'Co., 1960, p. 112.

Brady, G. S.: Materials Handbook, 9th ed., New York, McGraw-Hill, 1963, pp. 290-292.

ELLIOT A. GOLDBERG, Primary Examiner P0405) UNITED STATES PATENT OFFICE 5 6 CERTIFICATE OF CORRECTION Patent No. 3,463,872 Dated August 2 1969 Ihventor(s) Alvin Guttag It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Delete line 65 of column 2 in its entirety, which line to be deleted reads "metals having an atomic weight of less than 'rO, i.e.

dl Jnl-D SEALED FEB 1 71970 Q Am EdwardlLFlctcher, In WILLIAM I. swam, JR.

Gomissioner of Patents Meeting Officer 

